The present invention relates generally to an aircraft fuel tank, designed primarily for military aircraft, to minimize the damage caused by projectiles from enemy gunfire.
Fuel tanks in military aircraft present one of the most vulnerable components that can cause the disabling and/or destruction of the aircraft. This condition is particularly prevelant in low-flying operations where the aircraft is subject to gunfire from enemy ground troops using either hand guns or fixed installation weapons. Typical examples of these projectiles are 14.5 mm A.P.I. (Armor Piercing Incendiary) and 23 mm H.E.I. (High Explosive Incendiary). In-flight fires are by far the most frequently reported casualty (60-70%) of the combat losses on which reports are available. Aircraft fuel tanks hit by projectiles or shrapnel may be subjected to one or more of several casualties such as loss of fuel, fire, or explosion outside the punctured fuel tank in the surrounding structural cavity, and rupture of aircraft structure and fuel tank due to the transmission of the bullet kinetic energy in the form of hydrodynamic ram pressure which creates destructive shock waves. These casualties are particularly severe when a fuel tank wall is adjacent the engine inlet (normally where such fuel tank wall forms a common wall with the engine inlet wall). The hydrodynamic ram pressure can cause a rupture of the engine inlet duct dumping large quantities of fuel directly into the engine resulting in a flame-out, fire, or in the worst case, an explosion.
The phenomenon of hydrodynamic ram effects in liquids is well known and much has been developed to lessen the destructive effects of this phenomenon, especially in military combat aircraft fuel tanks. The standard self sealing tank will usually seal punctures caused by small caliber ammunition. However, the hydrodynamic ram effect created by these small caliber projectiles is minimal. Punctures caused by large caliber projectiles, such as 23 mm HEI, are not effectively sealed by state of the art fuel tanks. This factor, in conjunction with the significant hydrodynamic ram effect caused by the large caliber projectiles results in one or more of the aforementioned casualties.
Various attempts have been made to resolve the above problems by attenuating the hydrodynamic ram effect. However, none has shown success of consequence. One such design is to cover the tank exit wall with a rigidized crushable foam. Another technique is the use of various honeycomb sandwich constructions such as aluminum covered with either graphite epoxy or Kevlar.
Another such attempt is to place armor plate such as titanium on the exit wall of the fuel tank. However, this technique is both expensive and heavy and reflects the shock wave back to the entry wall which suffers extensive damage.